The invention relates to a process for the preparation of bisorganoalkaline earth metal compounds.
The invention further relates to a process for the polymerization of anionically polymerizable monomers using a bisorganoalkaline earth metal compound prepared by the aforementioned process as polymerization initiator.
The synthesis of bisorganoalkaline earth metal compounds by reacting bisorganomercury compounds with elemental calcium, strontium or barium has been described in U.S. Pat. No. 3,718,703. Although this synthesis route produces the bisorganoalkaline earth metal compounds in good yields, it is not desirable in view of the use of poisonous mercury compounds.
The metallation of CH-acidic organic compounds with calcium, strontium or barium in aprotic, polar solvents has been described in U.S. Pat. Nos. 3,965,080 and 4,012,336. In this way, it is possible, for example, to prepare dixanthenylbarium.
Russian Chemical Reviews, Vol. 50, 1981, p. 601-614, gives a review of other possible syntheses and the use of organoalkaline earth metal compounds in the anionic polymerization of unsaturated monomers. Some of the known syntheses for organoalkaline earth metal compounds are complex or produce the desired compounds in low yields or contaminated with by-products.
Burkey et al. in Organometallics 12 (1993), pages 1331-1337 describe solvent-free alkaline earth metal metallocenes solvated with tetrahydrofuran (THF) which are obtained by reacting potassium cyclopentadienide with an alkaline earth metal iodide.
It is an object of the present invention to provide an economical process for the preparation of bisorganoalkaline earth metal compounds in high yields and high purity using readily available and easy-to-handle starting compounds.
We have found that this object is achieved by a process for the preparation of bisorganoalkaline earth metal compounds which comprises reacting an organometallic compound RnM2 having a covalent metal-carbon bond content with an alkaline earth metal salt M1Xm.
We have also found a process for the polymerization of anionically polymerizable monomers using, as polymerization initiator, a bisorganoalkaline earth metal compound prepared by a process as claimed in any of claims 1 to 5.
The organometallic compounds which may be used for the novel process for the preparation of bisorganoalkaline earth metal compounds are any customary organometallic compounds having a covalent metal-carbon bond content. Preference is given to organolithium, organosodium, organopotassium or organomagnesium compounds, in particular organolithium or organomagnesium compounds, which are usually more soluble. Suitable organic groups R are preferably "sgr"-bonded hydrocarbons having from 1 to 25 carbon atoms, in particular C1-C25-alkyl, such as methyl, ethyl, n-propyl, isopropyl, n-butyl, isobutyl, tert-butyl, pentyl, hexyl, heptyl, octyl, nonyl, dodecyl, octadecyl, C6-C25-aryl, such as phenyl or substituted phenyl, such as 3,5-dimethylphenyl, p-t-butylphenyl, p-octylphenyl, p-dodecylphenyl, o-, m-, p-tolyl, biphenyl, naphthyl, aralkyl, such as benzyl, phenylethyl, 2-phenylpropyl, 6-phenylhexyl, p-methylphenylethyl, p-t-amylbenzyl, C3-C12-alkenyl, such as vinyl, allyl, 3-butenyl., 4-hexenyl, C3-C12-cycloalkyl, such as cyclopropyl, cyclobutyl, cyclopentyl, cyclohexyl, cyclooctyl, C3-C12-cycloalkenyl, such as cyclohexenyl or cyclooctenyl. The organic groups R may carry functional groups which are inert to the metal-carbon bond. Examples thereof are trimethylsilyl, trimethylsiloxy, ether, dialkylamino or cycloalkylamino groups. The integer n is 1 where M2 is an alkali metal, or 2 when M2 is magnesium. Particularly preferred organometallic compounds are benzyllithium and dibenzylmagnesium. The organometallic compounds are often in the form of adducts with solvents such as tetrahydrofuran, dioxane or complexing agents such as tetramethylethylenediamine or crown ethers. The preparation of the organometallic compounds is known per se. They can, for example, be prepared by reacting lithium or magnesium with the appropriate organohalides in organic solvents such as pentane, hexane or diethyl ether. Suitable alkaline earth metals M1 in the alkaline earth metal salts M1Xm are preferably calcium, strontium and barium. They are used in the form of their salts X as halides, amides, phosphides, alkyl oxides or aryl oxides, m being an integer 1 or 2 depending on the valency of X. Preference is given to the readily available bis(trimethylsilyl)amides or 2,4,6-tris(tert-butyl)phenoxides. The alkaline earth metal salts can be obtained, for example, by reacting the alkaline earth metals with the corresponding hydrohalic acids, amines, phosphines, alkyl hydroxides or aryl hydroxides with the elimination of hydrogen. Processes for the preparation of ether complexes by reacting calcium, strontium and barium in ethereal solution saturated with ammonia gas are described, for example, by S. R. Drake et al., Main Group Chemistry 14) 1991, page 243, Polyhedron (12) 1993, page 2307-2311 or Journal of the Chemical Society, Chemical Communications 1991, page 517-519.
The process is preferably used for the preparation of organoalkaline earth metal compounds R2M1 having two identical organic groups R, i.e. homoleptic alkaline earth metal compounds.
When using mixed organometallic compounds, for example butyloctylmagnesium, or mixtures of different organometallic compounds, heteroleptic bisorganoalkaline earth metal compounds are also available.
The novel reaction of the organometallic compounds RnM2 with the alkaline earth metal salts M1Xm is preferably carried out in an organic solvent or solvent mixture in which the bisorganoalkaline earth metal compounds produced have the lowest solubility product of all starting materials and products. Suitable solvents or solvent components are aliphatic or aromatic, aprotic solvents, such as aliphatic or cycloaliphatic ethers, for example dimethyl ether, diethyl ether, dibutyl ether, diisopropyl ether, dioxane or tetrahydrofuran, aliphatic and cycloaliphatic hydrocarbons, such as pentane, hexane, octane, cyclohexane or cyclooctane, or aromatic hydrocarbons, such as toluene or ethylbenzene.
The organometallic compound RnM2 is generally reacted with the alkaline earth metal salt M1Xm in stoichiometrically equivalent amounts.
In a preferred embodiment, the alkaline earth metal salt is dissolved in a polar, aprotic solvent, to which a solution of the organometallic compound is added dropwise, and the precipitate which forms is filtered off and washed.
The resulting bisorganoalkaline earth metal compound can be further purified where appropriate by the known methods of organometallic chemistry. For example, the bisorganoalkaline earth metal compound may be dissolved in a solvent and admixed or covered with a layer of a precipitant.
The temperature for the reaction is unimportant. It does of course depend on the melting points and boiling points of the solvents used, and also on the stability of the bisorganoalkaline earth metal compound. The reaction is usually carried out in the range from xe2x88x9280 to +50xc2x0 C., preferably in the range from 0 to 30xc2x0 C.
The bisorganoalkaline earth metal compounds prepared by the novel process are suitable as polymerization initiators for anionically polymerizable monomers such as dienes, styrene, acrylates, methacrylates, acrylonitriles and vinyl chloride. They are particularly suitable for the homo- and copolymerization of butadiene, isoprene and styrene.
To improve the thermal stability (the bisorganoalkaline earth metal compounds of lower alkyls are usually thermally unstable at room temperature) but also to improve the solubility, the bisorganoalkaline earth metal compounds may be reacted with 1,1-diphenylethylene. This is of particular importance if the polymerization is to be carried out in the absence of polar solvents.